• Winds are dominantly horizontal air motions on the earth’s surface. Wind is the air, which move in any direction parallel to the surface of the Earth.
• Pressure Belts: Two factors leading to the formation of high and low pressure are thermal and dynamic.
• A study of the distribution of air pressure reveals that the air pressure is not uniformly distributed over the Earth’s surface. In order to destroy this uneven distribution of pressure, winds are caused. In other words, winds are caused due to uneven distribution of pressure. Winds move from high to low pressure areas.
• Thermal Control: The sunrays make different angles at different latitudes. Where the rays are more vertical, the amount of insolations received is more, i.e., the place with more vertical rays is heated more than that where the sunrays are less vertical. Such a place, which is hot, heats the air with the result that the air pressure decreases. In this way many belts of high and low pressures are produced.
• Dynamic Control: The Earth is rotating about its axis. Due to this rotation the pressure belts shift from their ideal positions.  For example, the winds at poles shift towards equator.

DIRECTION OF WINDS

• Had the Earth not been rotating about its axis the winds would have blown in the direction of pressure gradient. The earth on account of its rotation produces a force known as Coriolis force after the name of the Mathematician Coriole. The force displaces the winds from the direction of wind(or pressure gradient. Ferrel formulated a law to deduce the direction of winds in the northern and southern hemispheres. It is known as Ferrel’s law.
• According to Ferrel’s law, if we stand with our faces in the direction in which the wind is blowing the wind will turn towards our right hand in the northern hemisphere and towards our left hand in the southern hemisphere.
• Buys Ballot’s Law: It relates the wind direction to the position of air pressure areas.
• According to this law, if we stand with our faces in the direction in which the wind is blowing we will find the area of low pressure to wards our left hand in the northern hemisphere and towards our right hand in the southern hemisphere.

PLANETARY WINDS

• The general permanent circulation of surface winds throughout the world is denoted by the term planetary winds. The wind belts are basically controlled by the latitudinal pressure belts and by the forces produced by rotation of the earth.

Doldrum

• It is the equatorial belt of variable winds and calms over the equatorial belt of low pressure lying between 5o south and 5o North latitude. This zone has no pressure gradients to induce a persistent flow of wind.

Trade winds

• The word ‘trade’ comes from the Saxon word tredon, which means to tread or follow a regular path. Moving north and south of the equator, the main wind belts are trade winds, covering roughly the zone between 5o and 30o north and south. They blow from the subtropical high-pressure areas (horse latitude) towards equatorial low-pressure areas (doldrums). Under the influence of the Coriolis force they blow from the northeast in the northern hemisphere (northeast trades) and from the southeast in the southern hemisphere (southeast trades). They are also called tropical easterlies.

Horse Latitudes

• Are the subtropical belts of variable winds and columns that lie between the latitudes 25o and 35o south and north. They coincide with the subtropical high-pressure belts. The high pressure is probably caused by the rising air of equatorial latitudes, which descends are.

Westerlies

• blow from subtropical high-pressure areas (Horse latitudes) to sub polar low-pressure areas and the between 35o and 60o N and S latitudes. Variable in direction and strength. Westerlies contain depressions. In the northern hemisphere, land masses cause considerable disruption to the Westerlies wind belt. But between 40o and 60o S, lies the almost unbroken ocean belt. Westerlies are strong and persistent here, giving rise to the mariner’s expressions, “the roaring forties”, “the furious fifties” and “the screaming sixties”.

Polar easterlies

• Constitute the wind system, characteristic of the arctic and polar zones. They blow from polar high-pressure areas to sub polar low-pressure areas.

Monsoon Winds

• Derived from the Arabic word ‘mausim’, meaning season. ‘Monsoon’ is applied to winds whose direction is reversed completely from one season to the next. Land masses of Asia and North America powerfully control the temperature and pressure conditions in the northern hemisphere. As pressure conditions control winds, these areas also develop wind system, quite independent of the belted wind system in the southern hemisphere.
• Summer Monsoon: During summer, a thermal’ or ‘heat’ low is developed over southern Asia in the lower levels of the atmosphere. It is a cyclone with a considerable airflow.
• Winter Monsoon: Reverse airflow from that of summers takes place in winter in Asia. The land area is dominated by a strong centre of high pressure, from which there is an outward flow of air. Blowing southward and southeastward towards the equatorial oceans, the winter monsoon brings dry, clear weather for several months.

Jet Streams

• The term was introduced in 1947, by Swedish born U.S. meteorologist Car Gustaf Rossby, and stands for a very strong steady westerly wind blowing at high altitudes (6,000 to about 1400 meters above the earth’s surface) just below the tropopause. It is usually confined to a narrow band and its speed reaches up to 350-450kph.
• The highest speed occurs during winter. There are two main jet streams: (a) polar front jet stream, irregular in its location and commonly discontinuous, (b) subtropical jet stream (between 20o and 30o latitudes, north and south), fairly consistent for a given season.

Local Winds

• These winds affect only limited area and blow for short periods of time, and are generated by immediate influences of the surrounding area. Most local winds are developed by depressions.Some types of local winds are discussed below.
• Land and Sea Breeze: The local wind that blows from sea to land during the day is called the sea breeze. During the night the land cools more quickly than the sea and a reverse process sets in. Land breeze is a cold wind that blows from the land to the sea (or large lake etc.).
• Mountain and Valley Winds: These are local winds, responding to local pressure gradients set up by heating or cooling of the lower air.
• Katabatic Winds: A cold downslope wind caused by the gravitational movement of cold dense air near the earth’s surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley. Favorable conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.
• Foehn and Chinook: These result when strong regional winds passing over a mountain range are forced to descend on the lower side with the result that the air is heated and dried.

FRONTS

• It is the zone along which two contrasting air masses meet, which originated in different source areas, therefore have differing temperatures and humidity characteristics. Best known fronts are Polar and the Intertropical Fronts.
• Depression is a mass of air whose isobars form an oval or circular shape, with low pressure at the centre. The air converges at the centre and rises to be disposed off. In a depression, the winds rotate anticlockwise in the northern hemisphere. While in the Southern hemisphere, the circular movement of winds is in a clockwise direction. Depressions are rarely stationary and tend to follow definite tracks. They are most influential over the ocean spreads and they weaken as they move over land areas. They are of two types:
• i.     Temperate Cyclones
• ii.    Tropical Cyclones

Temperate Cyclones

• They are found both on land and sea.
• Their isobars are usually V. shaped.
• They have a low pressure-gradient.
• The wind speed is low and never very strong.
• They occupy areas measuring thousands of square km.
• They travel from west to east.
• Rainfall is slow. Some times heavy showers take place. Rainfall continues for many days.
• More cyclones are produced in winter than in summer.

Tropical Cyclones

• They are produced and develop mainly over seas.
• Their isobars are usually complete circles. The pressure gradient is steep.
• The wind speed is about 100 km. per hour or even more.
• They have a small area.
• They travel from east to west.
• Rainfall is heavy. Rainfall does not last beyond a few fours. If cyclone stays at a place, the rainfall may continue for many days.
• The fronts are usually absent in tropical cyclones.
• The centre of the cyclone is known as ‘eye’. The winds at the eye are calm and there is no rainfall.

ANTICYCLONES

• This is a mass of air whose isobars also form an oval or circular shape, but in which pressure is high at the centre, decreasing towards the outside. Winds in an anticyclone form a clockwise out spiral in the northern hemisphere, whereas, they form an anticlockwise out spiral in the southern hemisphere.
• Classification of Anticyclones: According to Hanzlik anticyclones can be divided into two classes:
• 1.    Cold Anticyclones.
• 2.    Warm Anticyclones.